In the building construction industry there are many uses for posts whose length can be adjusted during and after construction. Examples include situations where the ground level is uneven, floor beams sag and need to be adjusted back to level, shoring elevated concrete form works, and where the post be placed on expansion soil, resulting in uneven elevations when the soil is moistened or dried.
Various prior art devices were developed to provide an extendable and retractable structure extending between two structure members. For example, U.S. Pat. No. 3,831,329 steel piers. The device has a small adjustable length, but cannot be used in vibrating conditions such as in supporting girders in the house. Another example is U.S. Pat. No. 3,027,140 which employs the same principle as U.S. Pat. No. 3,831,329. The above mentioned two patent and those listed on the reference prior arts have the same working principle—all of them move a thread rod up and down by adjusting the length of a post, forcing the thread rods to compress when holding weight. This limits the adjustable length of the post. The compression capacity will reduce with increasing length due to the stability effect. Other drawbacks of those devices include their inability to withstand dynamic loads, since their adjustment nut sits directly on top of lower part of the post, and the lack of restriction in either horizontal directions as well as upward direction.
The invention has radically different working principles as all prior art. When changing the length of the system, its adjustment thread rod undergoes tension, and after adjustment the system is restricted from movement in all directions, including horizontal and vertical. This alone provides substantial advantages over all prior arts. It can be used in anywhere a post support required or used as a jack at the same time. The maximum capacity of the system can match a steel rod's yield stress. The invention can be used in vibrating conditions such as houses.